Sweep circuit



Nov. 30, 1954 R. F. CASEY 2,695,955

SWEEP CIRCUIT Filed April 26, 1952 I2 I I \3 I6 68 INVENTOR. ROBERT F. CASEY ATTORNEYS United States Patent Office 2,695,955 Patented Nov. 30, 1954 SWEEP CIRCUIT Robert F. Casey, Pompton llains, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Clifton, N. .L, a corporation of Delaware Application April 26, 1952, Serial No. 284,610

7 Claims. (Cl. 250-27) This invention relates to electrical circuits, and particularly to circuits for producing sawtooth voltage waves. Such waves are useful, for example, in causing an electron beam to scan a phosphor screen in a cathode ray tube.

The invention is particularly useful when employed with cathode ray oscillographs which visually display images on the face of a cathode ray tube with respect to time. It is frequently desirable to expand certain portions of the time base in order to examine a portion of a displayed signal more minutely. The present invention provides a novel means for accomplishing this desideratum.

An object of the invention is to provide a sweep circuit which is capable of selectively expanding any por tion of its sawtooth shaped sweep voltage. Another object is to provide a high-speed selectively expandable sweep circuit.

Other objects and advantages of the invention will in part be obvious 'and in part appear hereinafter.

The invention comprises a sweep circuit for use in generating sawtooth voltage waves, the circuit being characterized by a plurality of condensers arranged in series, and a selectively operable discharge device in parallel with the series-connected condensers together with a second selectively-operable discharge device in parallel with one of said condensers, whereby it is rendered possible to generate sawtooth voltage signals and to expand substantially any selected portion of the signal. The invention accordingly is embodied in the circuit having the combinations .of elements and arrangements of parts hereinafter to be described in greater detail.

The single figure of the drawing is a schematic diagram of a preferred embodiment of the invention.

A source 11 of keying signals 12 is connected to a control electrode 13 of a discharge tube 14 having a cathode 16 which is grounded and an anode 17 which is connected to one end of a condenser 21 and to a control electrode 22 of an output tube 23 having an anode 24 connected to the voltage source 19 and a cathode 26 connected to an output terminal 27 and through a load resistance 28 to a source 29 of negative voltage. A resistance 30 is connected between the control electrode 13 and voltage source 19. The sawtooth wave forming condenser 21 is connected to the Voltage source 19 through a variable resistance 31 and a fixed resistance 32. A constant-voltage device 33 is connected between the output terminal 27 and the junction of the variable resistance 31 and fixed resistance 32.

An adjustable tap 36 on the load resistance 28 is connected through a resistance 37 to a control electrode 38 of a feedback tube 39 having a cathode 41 which is grounded and an anode 42 which is connected through a resistance 43 to the voltage source 19. The anode 42 is connected through a resistance 46 to a control electrode 47 of a second discharge tube or control tube 48 having a cathode 49 which is grounded and an anode 51 which is connected to the remaining end of the sawtooth wave forming condenser 21. The control electrode 47 is connected through a resistance 56 to the voltage source 29. The anode 51 of the control tube 48 is con nected to an expanded-sweep condenser 57, the remaining end of which is grounded, and to a cathode 58 of a restorer diode 59 having an anode 61 which is grounded, and to an anode 66 of a bypass diode 67 having a cathode 68 which is connected to an adjustable tap 71 of a resistance 72 which is connected between the voltage source 19 and ground.

The circuit operates as follows. The discharge tube 14 and the control tube 48 are normally conductive, thereby maintaining the sawtooth wave forming condenser 21 and the expanded sweep condenser '57 discharged.

When the keying signal 12 is made sufficiently negative in polarity, the discharge tube 14 ceases 'to be conductive, thereby allowing the sawtooth wave forming condenser 21 to commence to charge from the voltage source 19 through the fixed resistance32 and the variable resistance 31. As this condenser charges, it "provides an increasing voltage thereacross. This voltage is applied to the control electrode 22 of the output tube 23, which is connected in a cathode follower circuit; therefore, the increasing voltage at the condenser 21 provides an increasing output voltage at the output terminal 27. This increasing voltage at the output terminal 27 provides the normal sweep portions 75 and 79 of the output signal 76. The constant-voltage device 33, which may be, for example, a gas-filled tube or thyr-ite or the like, provides a feedback circuit for improving the linearity of the out put voltage as is more fully described'in my copending patent application, Serial No. 284,611. When the'keying signal 12 ceases to be negative as has been described, the discharge tube 14 returns to its normally conductive state and quickly discharges the sawtooth wave forming condenser 21, thereby providing a return sweep 77.

The expanded sweep portion of the circuit operates as follows. During the formation of-the normal sweep portion 75, this signal is applied to the control electrode 38 of the feedback tube 39, which tube inverts the polarity of the signal and applies a voltage increasing negatively to the control electrode 47 of the control tube 48. When this voltage becomes sufiiciently negative, the control tube 48 is rendered non-conductive, thereby placing the expanded sweep condenser 57 into the circuit in series with the sawtoooth wave forming condenser 21. This reduces the value of capacitance which is being charged from the voltage source 19 through the fixed resistance 32 and variable resistance 31. This reduced value of capacitance charges more quickly and, therefore, provides an expanded sweep portion 78 in the output signal 76. The transition point between the normal sweep 75 and the expanded sweep 78, is extremely sharp due to the feedback of the expanded sweep signal through the feedback tube 39. Since the expanded sweep signal 78 has a fast rate of voltage charge, it causes the tubes 39 and 48 to function quickly.

As the series-connected condensers 21 and 57 are being charged, during the expanded sweep portion of the output signal, the charging voltage across the expanded sweep condenser 57 is applied to the anode 66 of the bypass diode 67, the cathode of which is biased at a voltage depending on the adjustment of the tap 71 on the resistance 72. When the charging voltage of the condenser 57 reaches a value corresponding to the cathode bias voltage of the bypass diode 67, this diode becomes conductive and provides a path to ground from the bottom side of the sawtooth wave forming condenser 21 through a portion of the resistance 72. The resistance 72 preferably has a relatively low value of resistance with respect to the value of the charging resistance 31 and 32, so as not to appreciably affect the charging rate of the sawtooth wave forming condenser at this time during the operation of the circuit. The sawtooth wave forming condenser 21 now continues to charge at its original normal rate, thereby providing the second por tion 79 of the normal part of the sweep signal 76.

By adjusting the tap 36 on the output resistance 28, the time at which the expanded sweep 78 commences may be varied. By adjusting the position of the tap 71 on the resistance 72, the time at which the expanded sweep 78 ceases, can be varied. This controls the amplitude, or width, of the expanded-sweep signal. The restorer diode 59 functions to form part of the discharge path for the sawtooth wave forming condenser 21 at the end of each sweep; that is, the sawtooth wave forming condenser 21 is discharged at the end of each sweep J) through the series path comprising the discharge tube 14 and the restorer tube 59, the elements of these tubes being series-connected in such a manner as to provide the proper polarity for the discharge current to flow therethrough from the sawtooth wave forming condenser 21. The expanded-sweep condenser 57 also discharges fully or partially through this path. At the end of each sweep, the feedback tube 39 renders the control tube 48 conductive, thereby aiding in the discharging of the expanded-sweep condenser 57.

When the sawtooth wave forming condenser 21 and expanded sweep condenser 57 are thus discharged, they are in condition for forming a new sweep signal. During the second portion 79 of the normal output signal which occurs after the expanded portion 78 has been produced, the expanded sweep condenser 57 is maintained at a voltage determined by the bias voltage applied to the cathode 68 of the bypass diode 67.

The speed of all portions of the output sweep signal may be varied simultaneously by adjustment of the variable resistance 31, and by the characteristics of the constant-voltage device 33. The speed of the normal portions 75 and 79 of the sweep signal may be varied by adjustment of the size of the sawtooth wave forming condenser 21. The speed of the expanded sweep portion 753 of the output signal 76 may be readily adjusted by vary ing the size of the expanded sweep condenser 57. The negative voltage source 29 functions to provide proper operating voltages for the tubes in the circuit. The wave shape of the keying signal 12 determines the spacing of the non-sweep portions 81 of the output signal 76. If the keying signal 12 is made to have relatively narrow positively increasing portions, then the non-sweep portion 81 will be so short in time that the output will be, in effect, a continuous repetitive sawtooth wave in accordance with a repetitive keying signal 12. Single sawtooth output signals may be obtained by providing single keying pulses 12 having negative polarity.

While a preferred embodiment of the invention has been described, modifications thereof which will fall within the spirit and scope of the invention will be apparent to those skilled in the art. The scope of invention is defined in the following claims.

What is claimed is:

1. A sweep circuit comprising a plurality of condensers connected in series, a source of voltage, an impedance connected between said condensers and said source of voltage, a first selectively operable discharge device connected in parallel with said series-connected condensers, a second selectively operable discharge device connected in parallel with one of said condensers, and means connected to operate said second discharge device in accordance with the charge on the remaining said condensers.

2. A sweep circuit comprising a source of keying signals, a first discharge device connected to be actuated by said keying signals, a plurality of series-connected condensers connected in parallel with said first discharge device, a source of voltage, an impedance connected between said source of voltage and the junction of said series-connected condensers and said first discharge device, a cathode follower circuit having an input electrode connected to said junction and having an output load resistance, a second discharge device connected in parallel with one of said condensers, and having a control electrode, and coupling means connected between said output load resistance and said control electrode.

3. The circuit in accordance with claim 2, in which said coupling means includes a phase-inverting device.

4. The circuit in accordance with claim 2, in which the connection between said coupling means and said load resistance comprises an adjustable tap on said load resistance.

5. The circuit in accordance with claim 2, including a source of bias voltage, and a diode rectifier device connected between said source of bias voltage and the junction of said second discharge device and said one condenser.

6. The circuit in accordance with claim 2, including a diode rectifier device connected in parallel with said second discharge device.

7. The circuit in accordance with claim 1, including a rectifier and a source of bias voltage connected in series across said one of said condeners and polarized so as to provide a shunt path thereacross when the voltage thereacross is substantially equal to said bias voltage.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,140,840 Langer et a1 Dec. 20, 1938 2,250,819 Wolf July 29, 1941 2,284,101 Robins May 26, 1942 2,405,231 Newhouse Aug. 6, 1946 2,406,382 Kellogg Aug. 27, 1946 2,419,340 Easton Apr. 22, 1947 2,428,926 Bliss Oct. 14, 1947 2,441,246 Miller et a1. May 11, 1948 2,453,711 Isbister et al Nov. 16, 1948 

